1. Field of the Invention
The present invention relates generally to lithography systems. More particularly, the present invention relates to transferring and loading a reticle onto a receiving station while maintaining out-of-plane reticle alignment and providing rigidity of the reticle during the transfer.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. One skilled in the pertinent art would recognize that the description herein would also apply to other types of substrates known to those skilled in the pertinent art.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called a mask) for projecting the image onto the wafer.
The reticle is generally located between a semiconductor chip and a light source. In photolithography, the reticle is used as a photo mask for printing a circuit on a semiconductor chip, for example. The reticle, typically made of glass, requires delicate handling because it can easily be flawed by minute scratches or contamination.
A loading process is required for loading the reticle onto a reticle exposure stage. The reticle must be very precisely aligned at a predetermined position. Any error in the alignment must be within a correctable range of compliance. Accordingly, the distance between the reticle and the reticle exposure stage during the loading process must be minimized. The position of a reticle can be adjusted by rotating and moving the reticle in the X-Y plane direction. However, in minimizing this distance, careful attention must be paid to avoid flawing the reticle by bumping it against the stage.
Conventionally, an alignment mark formed in the reticle is used to align the reticle by locating the alignment mark. A reference mark in the reticle exposure stage is positioned under a reticle alignment microscope to assist in the alignment process. By detecting the alignment mark of the reticle through the microscope, pre-alignment and post-alignment operations are performed. To decrease reticle exchange time and thereby increase throughput of the lithography system, alignment of the reticle at an off-line alignment station is desirable.
Conventionally, “pushers” were used to pre-align the reticle in the off-line alignment station. Use of the pushers, however, resulted in non-compliance of the alignment of the reticle at the off-line alignment station with the required alignment of the reticle, which subsequently occurred at the reticle exposure stage. As a result, the throughput of the lithography system was still not increased.
The means of reticle transfer that could maintain compliant off-plane and remain rigid in-plane were not available prior to the present invention. In other words, prior to the present invention, the out-of-plane pre-alignment position alignment of the reticle at the off-line alignment station could not be maintained to the precision that is required to achieve the desired performance at the reticle exposure stage.
Therefore, what is needed is a method and apparatus of transferring and loading a pre-aligned reticle onto a reticle exposure stage without losing pre-alignment of the reticle, which previously occurred at the off-line alignment station. More specifically, what is needed is an apparatus and method for providing out-of-plane pre-alignment compliance with the alignment of the reticle at the reticle exposure stage, while maintaining in-plane rigidity of the reticle during transport to the alignment stage.